A fuel cell is an electrochemical device for continuously converting chemicals—a fuel and an oxidant—into direct-current electricity. It consists of two electronic-conductor electrodes separated by an ion-conducting electrolyte with provision for the continuous movement of fuel, oxidant and reaction product into and out of the cell. Fuel cells differ from batteries in that electricity is produced from chemical fuels fed to them as needed, so that their operating life is theoretically unlimited. Fuel is oxidized at the anode (negative electrode), giving electrons to an external circuit. Simultaneously with the electron transfer, an ionic current in the electrolyte completes the circuit. The fuels range from hydrogen and carbonaceous materials to redox compounds, alkali metals and biochemical materials. Fuel cells based on hydrogen and oxygen have a significant future as a primary energy source. Cells of this type are under development for use as a power source for electric automobiles, the hydrogen being derived from methanol, gasoline, diesel fuel or the like.
Fuel cells such as PEM fuel cells have been proposed for many applications including electrical power plants to replace internal combustion engines. PEM fuel cells are well known in the art and include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles (often supported on carbon particles) admixed with proton-conductive resin. The MEA is sandwiched between a pair of electrically-conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain channels for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode. In PEM fuel cells, hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant).
For vehicular applications, it is desirable to use a carbon-bound hydrogenous fuel (e.g., methane, gasoline, methanol, etc.). Such liquid fuels are particularly desirable as the source of the hydrogen used by the fuel cell owing to their ease of on-board storage and the existence of a nationwide infrastructure of service stations that can conveniently supply such liquids. These fuels must be dissociated to release their hydrogen content for fueling the fuel cell. The dissociation reaction is accomplished in a so-called “primary reactor” which is the first in a series of reactors comprising the fuel processor. Other reactors in the fuel processor serve to remove CO from the hydrogen produced by the primary reactor. One such known primary reactor for gasoline, for example, is a two-stage chemical reactor often referred to as an “autothermal reformer.” In an autothermal reformer (ATR), gasoline and water vapor (i.e., steam) are mixed with air and pass sequentially through two reaction sections, i.e., a first “partial oxidation” (POX) section and a second steam reforming (SR) section. In the POX section and with an open flame or a catalyst, the gasoline reacts exothermically with a substoichiometric amount of air to produce carbon monoxide, hydrogen and lower hydrocarbons such as methane. The hot POX reaction products, along with the steam introduced with the gasoline, pass into a SR section where the lower hydrocarbons and a fraction of the carbon monoxide react with the steam to produce a reformate gas comprising principally hydrogen, carbon dioxide and carbon monoxide. The SR reaction is endothermic, but obtains its required heat either from the heat that is generated in the exothermic POX section and carried forward into the SR section by the POX section effluent, or from other parts of the fuel cell system (e.g., from a combustor). One such autothermal reformer is described in International Patent Publication Number WO 98/08771, published Mar. 5, 1998.
Downstream of the ATR, the carbon monoxide contained in the SR effluent is removed, or at least reduced to very low concentrations (i.e., less than about 20 ppm) that are non-toxic to the anode catalyst in the fuel cell. To this end, fuel processors are known that cleanse the SR effluent of CO by first subjecting it to a so-called “water-gas-shift” reaction (i.e., CO+H2O→CO2+H2) followed by reacting it with oxygen (i.e., as air) in a so-called “preferential oxidation reaction” (i.e., CO+1/202→CO2). The CO-cleansed, H2-rich reformate is then supplied to the fuel cell.
Again, for an auto-thermal reformer, the air, fuel and steam must be mixed before entering the primary reactor. For system efficiency, it is desirable to integrate heat into these streams. However, at high temperatures (about 500° C. to 600° C.), the auto-ignition delay times of hydrocarbon fuels are relatively short (10 to 100 ms). If auto-ignition does occur before the mixture enters the primary reactor catalyst, these gas phase reactions will tend to form undesirable carbon deposits. The high temperatures can also cause pyrolysis of the fuel, leading to carbon formation. By utilizing a distributed injection device, the time required for mixing can be reduced, and the required mixing scale is small (based on the distance between the distributed injection points). Distributed injection can also be achieved by spray injection of liquid fuels as the fuel droplets can be dispersed over the cross-section of the inlet by the distribution and penetration of the fuel droplets. However, for an auto-thermal reformer requiring a large turndown, further enhancements may be required to achieve auto-ignition and carbon-free operation.
The present invention satisfies a need existing in the prior art and provides alternatives to and advantages over the prior art.